PROTOPTERUS. 



513 



mentation is complete and unequal, and is followed by 

 gastrulation. Segmentation of the embryo is obvious at a 

 very early period ; there is no trace of external gills. The 



early stages resemble very 

 closely the corresponding 

 stages in the development of 

 Amphibians. 



Ceratodus sometimes attains a 

 length of six feet. The body is 

 elongated and compressed, and bears 

 a continuous vertical fin. The paired 

 fins are trowel-like, with a median 

 jointed axis, from which rays pro- 

 ject on each side. There are four 

 gill clefts, four internal gills, and a 

 hyoid half gill. There are no ex- 

 ternal gills. 



The swim bladder or lung for 

 as such it acts is single. It is sup- 

 plied with blood from the fourth 

 aortic arches, as is the swim bladder 

 of the Ganoids Polypterus and 

 Amia. It arises ventrally, but lies 

 dorsally, and is divided into com- 

 partments. 



The heart has only one auricle, 

 with a dorsal fibrous ridge hinting 

 at a division. The conus arteriosus 

 is peculiarly twisted, and contains a 

 short longitudinal spiral valve and 

 numerous large " pocket " (or 

 " Ganoid ") valves. The septum 

 in the conus is not complete, as it 

 is in the other Dipnoi, thus mixed 

 blood passes into the first two pairs 

 of arches. There are four pairs of 

 these arches or arteries supplying 

 the gills ; the efferent vessels (two 

 from each gill, as in Elasmobranchs), 

 unite to form epibranchials, and 

 these to form the dorsal aorta. The 



fourth epibranchial gives off the pulmonary artery. The pulmonary 



vein enters the left side of the auricle. 



FIG. 173. Skeleton of Cera- 

 todus Fin. (From GEGEN- 

 BAUR.) 



., Central axis ; 

 basal piece. 



radial s 



(b} Protopterus. 



Protopterus lives in African rivers (Gambia, Quilimane, 

 &c.), is mainly but not exclusively carnivorous, and attains 

 33 



